1. Field of the Invention
The present invention relates to a semiconductor thin film decomposing method of decomposing an amorphous semiconductor thin film into a polycrystalline semiconductor thin film, a decomposed semiconductor thin film evaluation method, a thin film transistor made of a decomposed semiconductor thin film, and a semiconductor device including a flat panel type image display device having a circuit constituted of thin film transistors.
2. Description of the Related Art
A flat panel type image display device such as a liquid crystal display device has a pixel circuit and a drive circuit fabricated in a semiconductor thin film formed on an insulating substrate (hereinafter, also called simply a substrate), preferably a glass substrate. Thin film transistors (TFT) are often used in the pixel circuit and drive elements constituting the drive circuit. In place of an amorphous semiconductor thin film (typically, an amorphous silicon semiconductor thin film, or also called an a-Si film), a polycrystalline semiconductor thin film (typically a polysilicon semiconductor thin film, or also called a poly-Si film) has been used recently as the active layer of a thin film transistor so that an image of high definition and high quality can be displayed.
A semiconductor thin film used as the active layer of a thin film transistor will be described by taking a silicon semiconductor thin film by way of example. More excellent characteristics can be obtained by using a polycrystalline silicon semiconductor thin film as an active layer, than by using an amorphous silicon semiconductor thin film. One of the reasons for this may be ascribed to that a polycrystalline silicon semiconductor thin film has a higher mobility of carriers (electrons in an n-channel and holes in a p-channel) than that of an amorphous silicon semiconductor thin film. The cell size (pixel size) can therefore be reduced so that high definition is possible. Furthermore, a high temperature process at 1000° C. or higher is required to form a thin film transistor made of a general polysilicon semiconductor thin film. According to low temperature polysilicon semiconductor thin film forming techniques using annealing of only a silicon layer by a laser beam without heating a substrate supporting a semiconductor thin film to a high temperature, a thin film transistor (TFT) having a high mobility can be formed at a low temperature process, which allows an inexpensive glass substrate to be used.
Since the mobility generally becomes larger the larger the size of a crystal grain of a polysilicon semiconductor thin film is, a variety of technologies have been proposed for a method of forming a polysilicon semiconductor thin film having a large grain size. According to a general method, as described in JP-A-64-76715, a pulse laser beam shaped in a line beam having a trapezoidal intensity distribution profile is repetitively irradiated on an amorphous silicon semiconductor thin film, by shifting the line beam at a pitch of about 1/20 of a short axis width of the line beam in the unit of one shot. The amorphous silicon semiconductor thin film absorbs the irradiated laser beam so that the thin film raises its temperature and is melted to then lower the temperature. With this process, the silicon semiconductor thin film is crystallized so that the amorphous silicon semiconductor thin film is changed to a polysilicon semiconductor thin film (this phenomenon is called decomposition). The average grain size of a polysilicon semiconductor thin film changes with the energy density of an irradiated laser beam. At the energy density necessary for crystallizing an amorphous silicon semiconductor thin film or a lower energy density, the grain size becomes larger as the energy density is increased. However, at a certain threshold value, fine crystals having an average grain size of 100 nm or larger can be formed. It is therefore necessary that irradiation is performed at an energy density lower than the fine crystal threshold value.
In contrast with this, JP-A-9-246183 discloses a method of forming a large grain size at a skirt portion of an intensity distribution profile. This pulse laser irradiation area intensity distribution profile has a trapezoidal shape and a maximum intensity larger than the threshold value at which an amorphous silicon semiconductor thin film changes to a fine polysilicon semiconductor thin film.
The above-described techniques correspond to an average grain size of 1 μm. For example, if an amorphous silicon semiconductor thin film having a thickness of 50 nm is changed to a polysilicon semiconductor thin film, a margin of the laser energy density for obtaining a grain size of 0.3 μm or larger is about 10% of which about a half has a PV value of a protrusion of 70 nm or larger. The PV value is defined as a difference between the maximum and minimum values in a measurement range.
Another method of forming a large grain size of a silicon semiconductor thin film is a method of controlling an optical intensity distribution. One of these conventional techniques is a method called a sequential lateral solidification method (SLS method) as disclosed in Publication No. WO97/45827. According to this method, a laser beam intensity distribution in an in-plane direction is divided into a plurality of beam pieces having a micrometer size to form a temperature gradient in the in-plane direction and forcibly promote crystal growth in the lateral direction. With this method, protrusions are formed on the surfaces of boundaries where the directions of crystal growths of a plurality of crystal grains are collide with each other.
The position where the protrusion is formed corresponds to the light intensity peak position of each beam piece. The reason for this is that since this position has the highest temperature, crystallization progresses from both sides of the peak position toward the peak position and the crystal growths collide with each other at the peak position. This protrusion is higher than that of a polysilicon semiconductor thin film formed by irradiation of a laser beam shaped in the above-described trapezoidal profile, and this protrusion has the PV value of 100 nm or larger in some cases.
Methods proposed in JP-A-10-64815 and JP-A-2002-313724 are improved techniques of a method disclosed in JP-A-9-246183. According to this method, a laser beam is first irradiated at a high energy, and thereafter, crystallization is performed under an intensity distribution control of irradiation of a laser beam at an energy lower than the first high energy to form large crystal grains. It is described that this intensity distribution control method can obtain an asymmetrical intensity distribution by changing the positions of a focal point and a substrate surface. However, the cause and effect of the ability of obtaining the asymmetrical distribution shape is not disclosed. In contrast, JP-A-10-312963 describes that the trapezoidal shape is changed to an inverted bell shape by changing the positions of a focal point and a substrate surface, which contradicts the results described in JP-A-10-64815. Although JP-A-2000-11417 also describes that a similar irradiation distribution can obtain crystals having a large grain size, it does not describe a laser beam forming method.
The above-described conventional techniques use a pulse laser beam. The crystallization method for a silicon semiconductor thin film by such a pulse laser, i.e., the decomposition method, forms a polysilicon semiconductor thin film having a very large surface irregularity because protrusions are formed on the thin film after crystallization. It is therefore necessary to form a thick gate insulating film when, for example, a thin film transistor is to be fabricated on a polysilicon semiconductor thin film. There arises the problem that a transistor on-current reduces in inverse proportion with the thickness of a gate insulating film.
As an improved method of reducing protrusions of a polysilicon semiconductor thin film formed through irradiation of a laser line beam having a trapezoidal profile, JP-A-2000-353664 discloses a method of reducing protrusions by executing laser annealing at different energy densities a plurality of times. The contents disclosed in JP-A-2000-353664 are as follows. Namely, hydrogen contained in an amorphous silicon semiconductor thin film when it is formed is abruptly emitted during laser annealing and the film surface becomes rough. The film surface roughness can be prevented by emitting hydrogen before crystal annealing by irradiating a laser beam having a low energy density at a crystallization threshold value or lower.
As a protrusion reducing method by the SLS method, Publication No. WO01/71791 discloses a protrusion reducing method by which crystallization annealing is performed through laser beam irradiation and thereafter, the second laser beam irradiation is performed at 25% to 75% of the energy density realizing a perfect melting state.
The above-described methods have the following disadvantages. Namely, one disadvantage is a long time of an annealing process because a laser beam is irradiated at a plurality of stages and required to scan a substrate a plurality of times. Another disadvantage is irregular protrusion reduction effects on a substrate surface when a pulse laser is repetitively irradiated at a plurality of energy densities each for a different irradiation time to superpose each irradiated pulse laser beam, because the scanning speed is required to be changed to change the irradiation time because of a constant repetition period of the pulse laser beam.
Even if a polysilicon semiconductor thin film is formed by a method having effective protrusion reduction, a whole substrate surface roughness satisfying the management standards cannot be obtained unless some other process is incorporated, because there are a fluctuation of a laser output with time and an in-plane variation of an amorphous semiconductor thin film before crystallization. For example, this process inspects protrusions after crystallization, detects a high protrusion area or large surface roughness area, and re-crystallizes this area. A well-known technique of measuring the surface roughness is an evaluation method using an interatomic force microscope. However, this method takes at least about several minutes to evaluate even a fine area of 10 μm×10 μm so that it is unpractical to evaluate the surface roughness of a whole substrate.
As a method of evaluating a surface roughness at high speed, JP-A-11-274078 discloses an evaluation method using a surface glossiness (reflectance). This method is associated with the disadvantage that since the roughness is evaluated by a reflectance, there is interference caused by a thickness of a polysilicon semiconductor thin film and a thickness of a film between a glass substrate and thin film. As a method of evaluating at high speed an in-plane distribution of grain sizes of a polysilicon semiconductor thin film, JP-A-2003-109902 discloses a method of measuring a grain size by a spread with of an angular distribution of scattered light.